Plasma stabilizing apparatus employing feedback controls

ABSTRACT

A plasma stabilizing apparatus having circuits for measuring electron temperature and electron density of a plasma using triple probes, a plasma gas pressure control circuit, and a plasma excitation power control circuit, for automatically stabilizing the plasma.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a plasma source, and morespecifically to an apparatus for generating a plasma and for controllingcertain physical conditions of the plasma.

2. Description of Related Art

In recent years, the utility of plasma has expanded continually. Inparticular, in the field of production facilities for integratedcircuits (IC), a stable plasma source is in demand.

In the conventional high frequency plasma apparatus, adjustment of highfrequency or microwave power and gas pressure has been done manuallywith the consequence that it was not possible to follow automaticallyany fluctuations in the plasma conditions.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a device forgenerating a stable plasma.

The present invention is directed to a plasma stabilizer havingmeasuring circuits for measuring electron temperature and electrondensity of the plasma means of the triple probe; a gas pressure controlcircuit for controlling the plasma gas pressure with an output signal ofthe measured electron temperature; an electric power control circuit forcontrolling a plasma excitation power with a measured output signal ofthe electron density; and insulated coupling elements which connect themeasuring circuit, the gas pressure control circuit, and the powercontrol circuit, thereby automatically generating a stabilized plasma bycontrolling at least one of the plasma gas pressure and the plasmaexcitation power.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram of one embodiment of the present invention;

FIG. 2 is a system diagram of another embodiment thereof;

FIG. 3 is an explanatory diagram for the principle of the triple probemethod; and

FIG. 4 is a characteristic diagram of voltage V_(d2) versus electrontemperature Te.

FIG. 5 is a diagram showing an alternative to element 151 shown in FIG.1; and

FIG. 6 is another diagram showing another alternative to element 151shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is to provide a plasma stabilizer which is capableof generating stabilized plasma by first displaying directly on anindicator electron temperature and electron density of the plasma usingthe triple-probe method, and then, using these outputs, the highfrequency or microwave power quantity and the gas pressure arecontrolled, either independently or simultaneously.

In this case, the electron temperature and the electron density from themeasuring circuits must be taken out as voltage-to-ground outputs, sincethese circuits are floating from the standpoint of direct current. Withthis control voltage outputs, the high frequency or microwave power andthe gas pressure are controlled, thereby stabilizing plasma flame. PG,3

In case no insulated coupling element is used, the voltage-to-groundwhich corresponds to the measured output signals of the electrontemperature and the electron density are taken out of the abovementionedmeasuring circuit, with which the gas pressure control circuit and theelectronic control circuit may be regulated.

Since the output from the plasma characteristic detecting circuit by thetriple probe method is floating from the standpoint of direct current,it is necessary for taking the signals outside to insulatingly connectthe circuit in the direct current manner. The preferred embodiments ofthe present invention transmit control signals to an external circuit byuse of the insulated coupling elements such as an isolation amplifier, avoltage/frequency conversion circuit, a photo-isolator, and so forth,even if they are insulated from the standpoint of direct current; and asystem of taking out voltages-to-ground which correspond to those outputvoltages by means of electronic circuit. With these systems, an outputsignal of the electron temperature Te is introduced into a gas pressurecontrol circuit to control the gas pressure, and a high frequency ormicrowave output power is controlled with an output signal of theelectron density Ne, thereby automatically rendering the plasma flame tobe stable.

The electron density in the plasma is determined by a difference betweenthe number of electrons to be generated by ionization in a unit time andthe number of electrons to be extinguished by recombination. In anordinary plasma region where the electron energy is relatively low,ionization probability augments with increase in the electron energy,i.e., increase in high frequency or microwave excitation power to beimparted, while the recombination coefficient decreases, with theconsequence that the electron density Ne increases.

While electrons gain kinetic energy by the high frequency powerexcitation, they lose energy by their collision with atoms or molecules,a difference between the energy gain and the energy loss in a unit timedetermining the electron temperature Te.

These electron density Ne and electron temperature Te are both variedwith the high frequency or microwave excitation power as applied and thegas pressure. According to experiments conducted by direct reading ofthe electron density Ne and the electron temperature Te by the tripleprobe method, the electron density Ne indicated a large increase withincrease in the excitation power, but the electron temperature Teindicated just a slight increase. Also, the electron temperature Teindicated a remarkable decrease with increase in the gas pressure, whilethe electron density Ne showed a very small variation.

Therefore, in the preferred embodiments of the present invention, avoltage corresponding to an indicated voltage output of the electrontemperature Te was introduced into a gas control circuit to control thegas pressure by adjustment of an electromagnetic valve, while a voltagecorresponding to a voltage output of the electron density Ne wasintroduced into a power control circuit to control the high frequency ormicrowave output power, thereby stabilizing the plasma flame.

Although the automatic control by the gas pressure and the automaticcontrol by the high frequency or microwave power show theireffectiveness, even if any one of them is practised independently of theother, a more remarkable increase in stabilizing effect can be obtainedwhen both are carried out simultaneously.

The principle of, and measurement by, the triple probe method havealready been explained in various literatures in this field. Tosummarize, for the direct reading of Ne and Te, when three probes P₁,P₂, and P₃, each having an equal surface area, are inserted into theplasma in a mutually adjacent relationship, and a voltmeter (a Temeasuring circuit) having high input impedance is connected to a load ofthe probe P₂ as in FIG. 3 to render the probe to be in a floatingcondition, no electric current flows through the probe P₂. When, at thistime, the electric charge of the electron is expressed by "e",Boltzman's constant by "k", a difference voltage to be generated acrossthe probes P₁ and P₂ by V_(d2), and a constant voltage to be imparted tothe probe P₃ by V_(d3), so as to find a voltage drop of a current Iflowing from the probe P₃ to P₁ to appear at both ends of a low resistorR, the relationship will be expressed by the following equation.

    {1-exp (-φ.sub.d2)}/{1-exp (-φ.sub.d3)}=1/2

In the above equation,

    φ.sub.d2 =e V.sub.d2 /kTe

    φ.sub.d3 =e V.sub.d3 /kTe.

Accordingly, if V_(d3) is assumed to be a constant value, Te can beobtained by measurement of the value V_(d3).

This equation signifies that the ion current within the measuring rangehas small variations, although the ion current flowing in and through acylindrical probe in the collisionless plasma increases theoretically inproportion to a (1/2) power of the probing voltage. Hence, the ioncurrent I_(i) (V) at a voltage V can be expressed as follows, using aconstant β and with the ion current I_(i) (V_(f)) at a floating voltageV_(f) being made a reference.

    {I.sub.i (V)}.sup.2 ={I.sub.i (V.sub.f)}.sup.2 (1+βΔV)

Provided that the ion current varies in such manner, the electrontemperature Te with respect to various constants β will be as shown inFIG. 4, when V_(d3) =10 v constant.

According to experiments, it was found that, from the general plasmameasurement, when β=1.05, an error of Te stayed within a few per cents.

As to the electron density Ne, the following relationships have beenmade known:

    Ne=(M.sup.1/2 /S)·I·f.sub.1 (V.sub.d2)

    f.sub.1 (V.sub.d2)=1.05×10.sup.9 ×(Te).sup.-1/2 /}exp(-φ.sub.d2)-1}.

The units of measurement used are as follows: Ne (cm⁻³); the surfacearea of the probe (mm²); I (μ A); Te (eV); M (atomic quantity ormolecular quantity of ion); and V_(d2) (V). In the above two equations,if the values M and S are given, the electron density Ne can be obtainedfrom the probing current I, Te, and V_(d2), whereby the value of thiselectron density can be read directly on the output gauge.

An output voltage corresponding to the electron temperature Te as foundon the basis of the abovementioned principle is indicated on the gaugeand, at the same time, the direct current is interrupted by means of theinsulated coupling element, or an output voltage corresponding to theearthing point is found from the electronic circuit, the output voltageof which is amplified by the gas control circuit to control theelectromagnetic valve and to increase or decrease the gas pressure. Inmore detail, when the electron temperature Te is too high, the gaspressure is increased to lower Te, and, on the contrary, when Te is toolow, the gas pressure is reduced to elevate the Te.

On the other hand, the output voltage of the electron density Neindicated on the gauge is also found as a voltage corresponding to anoutput with respect to the earthing point, the output of which isintroduced into the excitation power control circuit to increase ordecrease the output power from the high frequency or microwavegenerating source, thereby controlling the plasma excitation power. Thatis to say, if the electron density Ne is excessively high, theexcitation power is lowered to reduce the density, and, on the contrary,if it is excessively small, the excitation power is increased to augmentthe electron density. In this way, plasma flame can be automaticallymaintained in a stabilized condition.

In the drawing, a numeral 1 refers to a plasma chamber: a numeral 2refers to a plasma flame; a numeral 3 refers to probes; a reference 4designates a gas inlet port; a numeral 5 denotes an electromagneticvalve: a reference numeral 6 designates an excitation power couplingport; a numeral 7 refers to a high frequency or microwave power source;a numeral 8 represents a Te measuring circuit; a numeral 9 refers to aTe output indicator; 10 designates an Ne measuring circuit; 11 denotesan Ne output indicator; 12 refers to a micro-resistor; 13 denotes aV_(d3) constant voltage power source: 14 represents an insulatedtransformer: 151 and 152 refer to insulated amplifiers; 16 denotes a gascontrol circuit; 17 indicates an excitation power control circuit; 191,192 and 193 designate resistors for preventing short-circuit; 201, 202and 203 denote input terminal resistors: 211, 212, 213 and 241, 242refer to differential amplifiers: 221, 222, 223, 231, 232, and 233designate inverters.

BEST MODE TO PRACTICE THE INVENTION

In the following, the plasma stabilizing device according to the presentinvention will be described in reference to the accompanying drawing.

FIG. 1 shows a system diagram for one embodiment of the presentinvention. A specimen gas is fed into the plasma chamber 1 through theelectromagnetic valve 5, while microwave power is introduced thereintothrough the coupling port 6 to thereby generate the plasma 2. Withinthis plasma, there are inserted three probes, each having an equalsurface area. As shown in the explanatory diagram of FIG. 3 for theprinciple of the triple probe method, the probe P₂ indicates a value ofTe on the indicator gauge 9 after it has been amplified in the electrontemperature Te measuring circuit 8. To the probe P₃, there is applied anegative voltage of 10 volts from the V_(d3) constant voltage source 13,and a voltage drop to be generated in the micro-resistor 12 of 1 ohmwith a current I flowing from the probe P₃ to the probe P₁ is calculatedin the Ne measuring circuit 10 together with the output voltage of Te todetermine a voltage corresponding to the electron density Ne, thequantity of Ne being indicated on the indicator gauge 11.

Since the output voltage from the Te measuring circuit 8 has to befloated in the form of a direct current, use is made of the insulatedamplifier 151 as the insulated coupling element, a control voltage beingapplied to the gas control circuit 16 in the subsequent stage to adjustthe electromagnetic valve 5, thereby controlling pressure of thespecimen gas.

As the insulated coupling element, use can be made of, besides theinsulated amplifier 151, an insulating method using a photo-insulator151' as shown in FIG. 5, or a voltage/frequency converter 151" as shownin FIG. 6; or a method, in which a direct current voltage is coupledwith an alternating current by means of an insulated transformer afterthe conversion, followed by returning it to the direct current, or othermethods.

Use was also made of the insulated amplifier 152 as the insulatedcoupling element to impart the output voltage from the Ne measuringcircuit 10 to the excitation power control circuit 17, wherein the powerof the microwave power source 7 of 2.45 GHz was adjusted to control theexcitation power to be imparted to the plasma, thereby stabilizing theplasma flame 2. The alternating current power necessary for thesecontrol operations is obtained by way of the insulated transformer 14.

As another embodiment of the present invention, FIG. 2 illustrates anextracted part which replaces the insulated coupling element shown inFIG. 1. When the output potential of the Te measuring circuit 8 isdesignated as V1, the output potential of the Ne measuring circuit 10 isdesignated as V2, and the potential at the positive (+) terminal of theV_(d3) constant voltage source is designated as V3, the Te outputindicator fluctuates in proportion to a voltage difference of V1 and V3,and the Ne output indicator fluctuates in proportion to a voltagedifference of V2 and V3.

Since the potential V1 is connected to the positive input terminal ofthe differential amplifier 211, the output of which is amplified by thetwo-stage inverters 221, 231, and the outputs from these inverters arefed back to the negative input terminal of the differential amplifier211, an equilibrated state is brought in when the output potential V1'of the inverter 231 becomes equal to the output potential V1.

The potentials at V2 and V3 are also amplified by those differentialamplifiers 212, 213 and inverters 222, 232, 223, 233 similar to thosementioned above, and the outputs V2' and V3' from them are equal to theinput potentials V2 and V3, respectively.

To the differential amplifier 241 at the subsequent stage, there areapplied the potentials of V1' and V3', hence the output therefrombecomes a voltage difference of V1' and V3', i.e., a voltage-to-groundcorresponding to the indicated voltage of Ne, so that an output voltageinsulated from the Te measuring circuit can be obtained without use ofthe insulated coupling element. This control voltage corresponding to Teis applied to the gas control circuit 16 to adjust the electromagneticvalve 5, thereby controlling the pressure of the specimen gas, andstabilizing the plasma flame.

Since the potentials of V2' and V3' are imparted to the differentialamplifier 242, the output therefrom becomes a voltage difference of V2'and V3', i.e., a voltage-to-ground corresponding to the indicatedvoltage of Ne, whereby it is possible to obtain an output voltageinsulated from the Ne measuring circuit without use of the insulatedcoupling element. This control voltage corresponding to Ne is applied tothe excitation power control circuit 17 to adjust the output power fromthe microwave power source and to control the plasma excitation power,thereby stabilizing the plasma flame.

In FIG. 2, reference numerals 191, 192, and 193 all designate resistorsfor preventing short-circuiting, which function to prevent, during aperiod until the differential amplifier circuit reaches its state ofequilibrium, any mal-effect due to lowering of the input load impedancefrom influencing on the preceding stage. Upon equilibration, however,the input load impedance becomes high, hence this voltage drop by theresistors does not bring about any problem. Reference numerals 201, 202,and 203 designate input terminal resistors.

INDUSTRIAL APPLICATION

The plasma contains therein very wide range of factors in itsinstability, all of which can not be removed. However, by putting thepresent invention into practice, remarkable improvement in them can beexpected. A fairly high degree of effect can be attained by carrying outthe automatic control of the gas pressure and high frequency waveexciting power, independently, according to the present invention. If,however, both of them are controlled simultaneously, the stability ofthe plasma could be improved by one numerical place of its value.According to experiments, a specimen gas prepared by mixing 10% ofmethane gas with hydrogen gas was used under a pressure of 0.2 Torr,while imparting to it a microwave excitation power of 300 W at 2.45 GHz,to generate plasma. This operation was continued for four hours, and itcould be continued very stably.

We claim:
 1. A plasma stabilizing apparatus for operating a system having three probes for immersion in a plasma, the apparatus comprising:a circuit for coupling to the three probes, for generating a first signal indicative of an electron temperature of the plasma and a second signal indicative of an electron density of the plasma; a coupling element, having a first input responsive to the first signal, a second input responsive to the second signal, and first and second outputs electrically isolated from the first and second inputs, for generating a third signal on the first output, the third signal being indicative of the electron temperature of the plasma, and for generating a fourth signal on the second output, the fourth signal being indicative of the electron density of the plasma; a circuit for controlling a plasma gas pressure in response to the third signal; and a circuit for controlling a plasma excitation power in response to the fourth signal.
 2. A plasma stabilizing apparatus for operating a system having first, second, and third probes for immersion in a plasma, the apparatus comprising:a circuit, for coupling to the first, second, and third probes, for generating a first signal indicative of an electron temperature of the plasma and a second signal indicative of an electron density of the plasma, the first circuit including a voltage source having an output coupled to the first probe; a subtracting circuit for generating a third signal, responsive to the first signal and to a voltage on the output of the voltage source, indicative of the electron temperature of the plasma, and for generating a fourth signal, responsive to the second signal and to the voltage on the output of the voltage source, indicative of the electron density of the plasma, a circuit for controlling a plasma gas pressure in response to the third signal; and a circuit for controlling a plasma excitation power in response to the fourth signal.
 3. The plasma stabilizing apparatus of claim 1, wherein the coupling element includes an isolation amplifier.
 4. The plasma stabilizing apparatus of claim 1, wherein the coupling element includes a voltage/frequency conversion circuit.
 5. The plasma stabilizing apparatus of claim 1, wherein the coupling element includes a photo-isolator. 